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Abstract:

A color image forming apparatus which is capable of performing color
misregistration correction, and registration of images and a recording
sheet while reducing the toner consumption amount. An image forming
section is caused to form a first reference image having a first density
for adjusting a density of an adjusting image, and a second reference
image having a second density different from the first density. A sensor
detects the first reference image and the second reference image. The
first density is determined as a density of the adjusting image, if an
output signal level of the sensor at a time of detection of the first
reference image is not lower than a predetermined value, and the output
signal level of the sensor at a time of detection of the second reference
image is lower than the predetermined value.

Claims:

1. An image forming apparatus comprising:an image forming section
configured to form an image on an image bearing member;a control section
configured to cause said image forming section to form an adjusting image
on the image bearing member; anda sensor configured to detect the
adjusting image,wherein said control section controls a position where
the image is formed, based on the result of detection by said sensor,
andwherein said control section causes said image forming section to form
a first reference image having a first density for adjusting a density of
the adjusting image, and a second reference image having a second density
different from the first density, causes said sensor to detect the first
reference image and the second reference image, and determines the first
density as a density of the adjusting image, if an output signal level of
said sensor at a time of detection of the first reference image is not
lower than a predetermined value, and the output signal level of said
sensor at a time of detection of the second reference image is lower than
the predetermined value.

2. A color image forming apparatus as claimed in claim 1, wherein if the
output signal level of said sensor at the time of detection of the first
reference image is not lower than the predetermined value, and the output
signal level of said sensor at the time of detection of the second
reference image is also not lower than the predetermined value, said
control section causes said image forming section to form a third
reference image having a third density lower than the first density and
the second density.

3. A color image forming apparatus as claimed in claim 1, wherein if the
output signal level of said sensor at the time of detection of the first
reference image is lower than the predetermined value, and the output
signal level of said sensor at the time of detection of the second
reference image is also lower than the predetermined value, said control
section causes said image forming section to form a third reference image
having a third density higher than the first density and the second
density.

4. A color image forming apparatus as claimed in claim 1, wherein said
control section causes said image forming section to form the adjusting
image at the determined density, and controls a position where the image
is to be formed, based on the result of detection of said sensor.

5. A color image forming apparatus comprising:an image forming section
configured to form an image on an image bearing member;a control section
configured to cause said image forming section to form an adjusting image
on the image bearing member; anda sensor configured to detect the
adjusting image,wherein said control section controls a position where
the image is formed, based on the result of detection by said sensor,
andwherein said control section causes said image forming section to form
a plurality of different reference images having respective different
densities for use in adjusting a density of the adjusting image, causes
said sensor to detect the reference images, and determine a density
corresponding to an output signal level of said sensor which is higher
than a predetermined value, and at the same time closest to the
predetermined value, as the density of the adjusting image.

6. A color image forming apparatus as claimed in claim 5, wherein said
control section causes said image forming section to form the adjusting
image at the determined density, and controls a position where the image
is to be formed, based on the result of detection of said sensor.

7. A method of controlling an image forming apparatus including an image
forming section configured to form an image on an image bearing member, a
control section configured to cause the image forming section to form an
adjusting image on the image bearing member, and a sensor configured to
detect the adjusting image, wherein the control section controls a
position where the image is formed, based on the result of detection by
the sensor, comprising:a reference image-forming step of forming a first
reference image having a first density for adjusting a density of the
adjusting image, and a second reference image having a second density
different from the first density;a detection step of detecting the first
reference image and the second reference image; anda determining step of
determining the first density as a density of the adjusting image, if an
output signal level of the sensor at a time of detection of the first
reference image is not lower than a predetermined value, and the output
signal level of the sensor at a time of detection of the second reference
image is lower than the predetermined value.

8. A method as claimed in claim 7, wherein if the output signal level of
the sensor at the time of detection of the first reference image is not
lower than the predetermined value, and the output signal level of the
sensor at the time of detection of the second reference image is also not
lower than the predetermined value, the control section causes the image
forming section to form a third reference image having a third density
lower than the first density and the second density.

9. A method as claimed in claim 7, wherein if the output signal level of
the sensor at the time of detection of the first reference image is lower
than the predetermined value, and the output signal level of the sensor
at the time of detection of the second reference image is also lower than
the predetermined value, the control section causes the image forming
section to form a third reference image having a third density higher
than the first density and the second density.

10. A color image forming apparatus as claimed in claim 7, wherein the
control section causes the image forming section to form the adjusting
image at the determined density, and controls a position where the image
is to be formed, based on the result of detection of the sensor.

11. A method of controlling an image forming apparatus including an image
forming section configured to form an image on an image bearing member, a
control section configured to cause the image forming section to form an
adjusting image on the image bearing member, and a sensor configured to
detect the adjusting image, wherein the control section controls a
position where the image is formed, based on the result of detection by
the sensor, comprising:a reference image-forming step of forming a
plurality of different reference images having respective different
densities for use in adjusting a density of the adjusting image;a
detection step of detecting the reference images; anda determining step
of determining a density corresponding to an output signal level of the
sensor which is higher than a predetermined value, and at the same time
closest to the predetermined value, as the density of the adjusting
image.

12. A method as claimed in claim 11, wherein the control section causes
the image forming section to form the adjusting image at the determined
density, and controls a position where the image is to be formed, based
on the result of detection of the sensor.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a color image forming apparatus
that forms a color image using an electrophotographic method, and a
control method therefor.

[0003]2. Description of the Related Art

[0004]Recently, image forming apparatus, such as copying machines
printers, are demanded to form high-quality images, at high speed with
high accuracy. Particularly, for color image forming apparatuses, as a
means for increasing the speed, there have been employed, for example, a
tandem method in which for a plurality of colors (the four colors of
yellow, magenta, cyan, and black), respective image forming units are
provided for sequential image forming operations. Each image forming unit
performs image formation by an electrophotographic process including
electrostatic charging, exposure, development of an image, and transfer
of the image to a sheet.

[0005]The tandem-type color image forming apparatus is advantageous in
that it can attain high printing speed, but is disadvantageous in that it
is difficult to reduce color misregistration when superposing a plurality
of color images one upon another. Even if the color misregistration
correction is carried out immediately after the image forming apparatus
is installed, misregistration is produced as time elapses, and even if
the color misregistration correction is carried out immediately before
the image forming apparatus is used, subtle displacement occurs due to a
change in temperature. Such displacement causes color misregistration in
a color image formed by superposition of color images.

[0006]To solve the problem, there have recently been proposed lots of
techniques for preventing such color misregistration. For example, a
position-detecting patch is formed on a transfer belt for detecting color
misregistration, and the patch is detected by a CCD line sensor to
thereby detect color misregistration of each color (see e.g. Japanese
Patent Laid-Open Publication No. H06-18796). Further, two or more
position-detecting patches are detected by optical sensors to thereby
detect color misregistration of each color (see e.g. Japanese Patent
Laid-Open Publication No. H06-118735).

[0007]The means for detection of position-detecting patches to correct
color misregistration executes the detection every certain
(predetermined) number of sheets or at certain (predetermined) time
intervals, and position detecting data obtained by reading the patches of
all the colors are averaged for use in color misregistration correction.
The position-detecting patches are basically formed by patches with
maximum densities, and if the large number of patches are required,
larger amounts of toners (developing agents) are consumed, and further,
an increased burden is laid on cleaning members for collecting the toners
since an image of the patches is not transferred to a sheet.

[0008]Now, a technique has been proposed which uses density sensors for
detecting toner densities also as position-detecting sensors for
detecting color misregistration, with a view to reducing the amount of
toner consumed for color misregistration correction (see e.g. Japanese
Patent Laid-Open Publication No. 2004-109682).

[0009]However, the proposed technique of using the density sensors for
detecting toner densities also as the position-detecting sensors for
detecting color misregistration suffers from the problem that the density
sensors cannot provide sufficient accuracy in detecting color
misregistration.

SUMMARY OF THE INVENTION

[0010]The present invention provides a color image forming apparatus which
is capable of performing color misregistration correction, and
registration of images and a recording sheet while reducing the toner
consumption amount.

[0011]In a first aspect of the present the present invention, there is
provided an image forming apparatus comprising an image forming section
configured to form an image on an image bearing member, a control section
configured to cause the image forming section to form an adjusting image
on the image bearing member, and a sensor configured to detect the
adjusting image, wherein the control section controls a position where
the image is formed, based on the result of detection by the sensor, and
wherein the control section causes the image forming section to form a
first reference image having a first density for adjusting a density of
the adjusting image, and a second reference image having a second density
different from the first density, causes the sensor to detect the first
reference image and the second reference image, and determines the first
density as a density of the adjusting image, if an output signal level of
the sensor at a time of detection of the first reference image is not
lower than a predetermined value, and the output signal level of the
sensor at a time of detection of the second reference image is lower than
the predetermined value.

[0012]According to the color image forming apparatus according to the
first aspect of the present invention, the control section causes the
image forming section to form an adjusting image on the image bearing
member. The sensor detects the adjusting image. The control section
causes the image forming section to form a first reference image having a
first density for adjusting a density of the adjusting image, and a
second reference image having a second density different from the first
density, causes the sensor to detect the first reference image and the
second reference image, and determines the first density as a density of
the adjusting image, if an output signal level of the sensor at a time of
detection of the first reference image is not lower than a predetermined
value, and the output signal level of the sensor at a time of detection
of the second reference image is lower than the predetermined value.

[0013]With this arrangement, it is possible to perform color
misregistration correction, and registration of images and a recording
sheet while reducing the toner consumption amount.

[0014]If the output signal level of the sensor at the time of detection of
the first reference image is not lower than the predetermined value, and
the output level of the sensor at the time of detection of the second
reference image is also not lower than the predetermined value, the
control section can cause the image forming section to form a third
reference image having a third density lower than the first density and
the second density.

[0015]If the output signal level of the sensor at the time of detection of
the first reference image is lower than the predetermined value, and the
output level of the sensor at the time of detection of the second
reference image is also lower than the predetermined value, the control
section can cause the image forming section to form a third reference
image having a third density higher than the first density and the second
density.

[0016]The control section can cause the image forming section to form the
adjusting image at the determined density, and can control a position
where the image is to be formed, based on the result of detection of the
sensor.

[0017]In a second aspect of the present invention, there is provided a
color image forming apparatus comprising an image forming section
configured to form an image on an image bearing member, a control section
configured to cause the image forming section to form an adjusting image
on the image bearing member, and a sensor configured to detect the
adjusting image, wherein the control section controls a position where
the image is formed, based on the result of detection by the sensor, and
wherein the control section causes the image forming section to form a
plurality of different reference images having respective different
densities for use in adjusting a density of the adjusting image, causes
the sensor to detect the reference images, and determine a density
corresponding to an output signal level of the sensor which is higher
than a predetermined value, and at the same time closest to the
predetermined value, as the density of the adjusting image.

[0018]The control section can cause the image forming section to form the
adjusting image at the determined density, and controls a position where
the image is to be formed, based on the result of detection of the
sensor.

[0019]In a third aspect of the present invention, there is provided a
method of controlling an image forming apparatus including an image
forming section configured to form an image on an image bearing member, a
control section configured to cause the image forming section to form an
adjusting image on the image bearing member, and a sensor configured to
detect the adjusting image, wherein the control section controls a
position where the image is formed, based on the result of detection by
the sensor, comprising reference image-forming step of forming a first
reference image having a first density for adjusting a density of the
adjusting image, and a second reference image having a second density
different from the first density, a detection step of detecting the first
reference image and the second reference image, and a determining step of
determining the first density as a density of the adjusting image, if an
output signal level of the sensor at a time of detection of the first
reference image is not lower than a predetermined value, and the output
signal level of the sensor at a time of detection of the second reference
image is lower than the predetermined value.

[0020]In a fourth aspect of the present invention, there is provided a
method of controlling an image forming apparatus including an image
forming section configured to form an image on an image bearing member, a
control section configured to cause the image forming section to form an
adjusting image on the image bearing member, and a sensor configured to
detect the adjusting image, wherein the control section controls a
position where the image is formed, based on the result of detection by
the sensor, comprising a reference image-forming step of forming a
plurality of different reference images having respective different
densities for use in adjusting a density of the adjusting image, a
detection step of detecting the reference images, and a determining step
of determining a density corresponding to an output signal level of the
sensor which is higher than a predetermined value, and at the same time
closest to the predetermined value, as the density of the adjusting
image.

[0021]The features and advantages of the invention will become more
apparent from the following detailed description taken in conjunction
with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a view of the whole arrangement of a color image forming
apparatus according to an embodiment of the present invention.

[0023]FIG. 2 is a schematic diagram showing an example of a
position-detecting patch formed on an intermediate transfer belt of the
color image forming apparatus in FIG. 1.

[0024]FIG. 3 is a diagram showing a position-detecting patch and the
waveform of an output voltage from a first patch sensor which indicates
the detected position-detecting patch.

[0025]FIG. 4 is a diagram showing an image leading edge-detecting patch,
and the relationship between the waveform of an output voltage from a
second patch sensor which indicates the detected image leading
edge-detecting patch.

[0026]FIG. 5 is a diagram of the arrangement of the first patch sensor
appearing in FIG. 2.

[0027]FIG. 6 is a flowchart of a first patch density-adjusting process
executed by the color image forming apparatus shown in FIG. 1.

[0028]FIG. 7 is a flowchart of a second patch density-adjusting process
executed by the color image forming apparatus shown in FIG. 1.

[0029]FIG. 8 is a schematic diagram showing an example of an image leading
edge-detecting patch formed on the intermediate transfer belt of the
color image forming apparatus in FIG. 1.

[0030]FIG. 9 is a schematic functional block diagram of the color image
forming apparatus according to the present embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031]The present invention will now be described in detail with reference
to the drawings showing preferred embodiments thereof.

[0032]FIG. 1 is a view of the whole arrangement of a color image forming
apparatus according to an embodiment of the present invention.

[0033]In FIG. 1, an apparatus main unit 1 of the image forming apparatus
includes image forming units for a plurality of colors (the four colors
of yellow, magenta, cyan, and black). The image forming units has
photosensitive drums 2a to 2d as image bearing members, and around the
photosensitive drums 2a to 2d, there are arranged electrostatic chargers
3a to 3d, cleaners 4a to 4d, laser scanning units 5a to 5d, transfer
blades 6a to 6d, and developing units 7a to 7d, in a manner opposed to
the respective outer peripheral surfaces thereof.

[0034]Below the image forming units, there extends an intermediate
transfer belt 8 whose upper surface is flat and is in contact with the
photosensitive drums 2a to 2d. The intermediate transfer belt 8 is
supported by rollers 10 and 11, and a cleaner 12 is in contact with the
intermediate transfer belt 8. The image forming apparatus including four
image forming units provided on the horizontal surface of the
intermediate transfer belt 8 is called a tandem type.

[0036]Then, the toner images of the respective colors developed on the
photosensitive drums 2a to 2d are formed as a four-color toner image on
the intermediate transfer belt 8, and are collectively transferred to the
recording sheet S by the secondary transfer roller 22. The recording
sheet then passes through the fixing unit 23 whereby the toners are
melted to form a permanent image.

[0037]On the other hand, the recording sheet S is fed from the sheet feed
cassette 17 or the manual feed tray 13, and has its lateral registration
position corrected by an electrostatic conveying unit 30. Then, the
recording sheet S is conveyed to the secondary transfer roller 22 while
making the registration timing synchronous by the registration rollers
16.

[0038]At this time, the recording sheet conveying-component parts, such as
the pickup rollers 18 and 19, the vertical path rollers 20, the
registration rollers 16, and the pickup rollers 14 and 15 are driven by
stepping motors independently of each other, so as to realize a
high-speed stable conveying operation.

[0039]Further, when performing double-sided printing, the recording sheet
S having passed through the fixing unit 23 and the registration rollers
24 is guided to the double-sided inverting path 27, wherein it is
inverted and conveyed in an opposite direction into the double-sided path
28. After passing through the double-sided path, the recording sheet S
passes through the vertical path rollers 20 again, and similarly to a
first-side image already formed, a second-side image is formed and
transferred to the recording sheet S. Then, after having the second-side
image fixed thereto, the recording sheet S is discharged.

[0040]FIG. 2 is a schematic diagram showing an example of a
position-detecting patch formed on the intermediate transfer belt of the
color image forming apparatus in FIG. 1.

[0041]In the tandem-type color image forming apparatus, as described
hereinabove, a plurality of toner images of yellow, magenta, cyan, and
black formed on the respective photosensitive drums 2a to 2d are
sequentially transferred onto the intermediate transfer belt 8 in an
superimposed manner. The toner images of yellow, magenta, cyan, and black
transferred in a superimposed manner are finally collectively transferred
onto the recording sheet S to thereby form a color image on the recording
sheet S.

[0042]Color patches of the position-detecting patch 41 for color
registration are formed by stations (image forming units) for the colors,
and are transferred to the intermediate transfer belt 8, and the
position-detecting patch 41 thus formed is read by a first patch sensor
40.

[0043]FIG. 3 is a diagram showing a position-detecting patch and the
waveform of an output voltage (output signal) (analog) from the first
patch sensor 40 which indicates the detected position-detecting patch.
FIG. 4 is a diagram showing an image leading edge-detecting patch, and
the relationship between the waveform of an output voltage (output
signal) from a second patch sensor 44 which indicates the detected image
leading edge-detecting patch and a reference value. These figures will be
referred to hereinafter.

[0044]FIG. 5 is a diagram showing the arrangement of the first patch
sensor 40 appearing in FIG. 2.

[0045]As shown in FIG. 5, the first patch sensor 40 has a light emitting
section and a light receiving section 53. Light emitted from the light
emitting section 52 hits the intermediate transfer belt 8 and the
position-detecting patch 41, and light reflected therefrom enters the
light receiving section 53, where the light is subjected to photoelectric
conversion, which gives an output voltage commensurate with the amount of
the light. Lenses 54a and 54b are disposed respectively between the light
emitting section 52 and the light receiving section 53 and an object to
be detected, such as the intermediate transfer belt 8. The lens 54a
causes the light emitted from the light emitting section 52 to converge,
while the lens 54b causes the reflected light to be efficiently received
by the light receiving section 53.

[0046]The first patch sensor 40 is configured to be capable of adjusting
the amount of light, based on the output voltage of the light receiving
section 53 that receives the light reflected from the background of the
intermediate transfer belt 8, such that a certain appropriate output
voltage is obtained. To cope with an unintended variation in the amount
of light, the first patch sensor 40 changes the amount of light emitted
therefrom to thereby adjust the amount of emitted light such that the
output voltage thereof becomes equal to a predetermined value.

[0047]The adjustment of the amount of light is generally executed under
the condition of no position-detecting patch 41 being formed on the
intermediate transfer belt 8, i.e. using the base of the intermediate
transfer belt 8. The waveform of an output signal (analog) 50 occurring
when the position-detecting patch 41 is read after adjustment of the
amount of light becomes e.g. as shown in FIG. 5.

[0048]The output voltage (output signal waveform 50) of the base of the
intermediate transfer belt is configured to be equal to a certain
predetermined value (e.g. 5V) after adjustment of the amount of light. In
the case of the first patch sensor 40 of a regular reflection type, the
output voltage thereof upon detection of the position-detecting patch 41
becomes lower than the output voltage upon detection of the base of the
intermediate transfer belt 8, and the output voltage and a threshold
value (reference value) 55 with reference to which the output voltage can
be normally converted into a digital signal are adjusted such that the
former become equal to or lower than the latter upon detection of the
position-detecting patch 41.

[0049]The baycener of a rise and a fall of the resulting digitized output
signal waveform 51 is determined, and is used as position detecting data
of the position-detecting patch 41. The sensor output (waveform 61) upon
detection of the yellow, magenta, cyan, and black color patches of the
position-detecting patch 41 formed by the respective stations are as
shown in FIG. 3. The respective reflectance coefficients of yellow,
magenta, cyan and black toners are different therebetween, and hence the
output voltages for the respective color patches are different from each
other. This is because the reflectance coefficient varies with the color
and density of the toner. It should be noted that the second patch sensor
44 also has the same arrangement as that of the first patch sensor 41.

[0050]In the present embodiment, the density of the position-detecting
patch 41 is adjusted for color misregistration correction. Before that,
first, the amount of light is adjusted using the base of the intermediate
transfer belt 8 while conveying the transfer belt 8. Then, after the
output voltage becomes equal to the predetermined value, the amount of
light is kept fixed until the amount of light is adjusted next time. The
timing for the adjustment of the amount of light may be set to each time
point when the apparatus is started up or to each time point when the
number of printed sheets has reached a predetermined value.

[0051]Next, the density adjustment of the position-detecting patch 41 is
carried out. Color patches formed by the respective stations (image
forming units) are transferred to the intermediate transfer belt 8 to
form the position-detecting patch 41. The patch densities are provided in
a plurality of levels ranging from a lowest density to a highest density.

[0052]FIG. 6 is a flowchart of a first patch density-adjusting process
executed by the color image forming apparatus shown in FIG. 2.

[0053]As shown in FIG. 6, first, a position-detecting patch (hereinafter
also simply referred to as "the patch") 41 at a toner density of 100% is
formed (step S601), and the first patch sensor 40 reads (sequentially
detect) the color patches of the patch 41 (step S602).

[0054]It is determined whether or not the output voltage of the first
patch sensor 40 has reached a predetermined value (step S603). If the
patch-reading voltage has not reached the predetermined value, it is
determined that a sensor error has occurred (step S606). As a method of
this determination, it is envisaged to employ a method of binarizing the
analog signal using a comparator, and determining whether the binary data
indicates a low value Lo.

[0055]If the output voltage has reached the predetermined value, the patch
41 with a toner density lowered by a predetermined percentage (e.g. a
toner density of 90%) is formed and read (step S604). Then, it is
determined whether or not the output voltage has reached the
predetermined value (step S605).

[0056]If the output voltage has not reached the predetermined value, the
patch having the density at the preceding level is set to the
position-detecting patch 41. That is, the patch density is set to the
immediately preceding higher density (step S607), and color registration
detection control (color misregistration correction control) is executed
(step S608), followed by terminating the present process. If the output
voltage has reached the predetermined value, the patch with a toner
density further lowered by a predetermined percentage is formed and read
(step S604). This sequence of operations is repeatedly carried out to
determine an appropriate patch density.

[0057]Although in the above-described method, one patch is formed for one
density to determine whether the patch detection level shows an
appropriate value, a plurality of patches with different densities may be
formed, and the patches 41 with different densities are continuously read
to sequentially determine on a density basis whether the output voltage
gives an appropriate value, as described hereafter with reference to FIG.
7.

[0058]FIG. 7 is a flowchart of a second patch density-adjusting process
executed by the color image-forming apparatus in FIG. 2.

[0059]As shown in FIG. 7, a plurality of patches with different densities
are generated (step S701), and read (step S702). Insofar as the output
voltage of the first patch sensor 40 has an appropriate value, a variable
N indicative of the number of times of patch detection is incremented by
1 (steps S703 and S704), to determine whether or not the output voltage
associated with an N-th number path has an appropriate value (step S705).

[0060]If the output voltage has an appropriate value, and the variable N
indicative of the number of times of the patch detection is not larger
than the number of the generated patches (NO to the step S706), the
process returns to the step S704. If the variable N indicative of the
number of times of the patch detection is larger than the number of the
generated patches (YES to the step S706), it is judged that a sensor
error has occurred, and a density selection sequence in which the
densities of patches are different from those of the patches formed
before is executed again, or an error message is displayed.

[0061]If the output voltage associated with the N-th patch does not have
an appropriate value, it is determined whether or not N=1 holds (step
S707). That is, it is determined whether or not the output voltage of the
first patch sensor 40 delivered upon reading a patch having the highest
density has an appropriate value. If the value read for the first time is
not an appropriate value, i.e. if N=1 holds (YES to step S707), it is
judged that a sensor error has occurred, whereas if N=1 does not hold,
the density of the N-1-th patch is stored in a storage device (step
S708). The color registration detection control is executed at this
density (step S709), followed by terminating the present process.

[0062]It should be noted that in FIG. 6, the patches are generated
starting with a patch with a density of 100% (density: 1.6), but they may
be generated starting with a patch with a density of 1.4. In this case,
it is possible to detect patch densities using a smaller number of
generated patches than the number of patches generated in FIG. 6.

[0063]Further, the patches are sequentially generated in the order of
decreasing densities, but they may be sequentially generated in the order
of increasing densities, whereby not only the amount of toners consumed
for executing color registration, but also the amount of toners consumed
at a stage of determining a patch density can be reduced, and the burden
on the cleaners can be lessened.

[0064]In this case as well, without generating patches starting with a
density of 0.1, but by starting with a density, e.g. 0.8, which is close
to a threshold where the value changes from outside an appropriate range
into the appropriate range, to progressively increase the density, it is
possible to detect patch densities by minimizing the number of generates
patches. In this case, a value of the density which results in a shift
from outside the appropriate range into the appropriate range, e.g. 1.0
is caused to be set to the density of the patch.

[0065]In the present embodiment, sensor error detection is performed using
the patch formed first, if the patches are formed starting with a patch
outside the appropriate density range, the first patch alone may be
formed such that it necessary has a density within the appropriate range,
and after confirming that a sensor error does not occur, the following
patches may be formed as described above while changing the density in a
direction from outside the appropriate range into the appropriate range,
starting with a patch with a density lower than the appropriate values.

[0066]The sequence of reducing the toner consumption amount can be applied
not only to the position-detecting patch 41, but also to the image
leading edge-detecting patch 42 for use in leading edge registration
control for registration between an image and the recording sheet.

[0067]FIG. 8 is a schematic diagram showing an example of an image leading
edge-detecting patch formed on the intermediate transfer belt of the
color image forming apparatus in FIG. 1.

[0068]As shown in FIG. 8, the image leading edge-detecting patch 42 is
formed on the intermediate transfer belt 8. The image leading
edge-detecting patch 42 is formed at a position a predetermined distance
forward of the image 43 to be transferred onto the recording sheet S.
Further, the image leading edge-detecting patch 42 is generated outside
the recording area of the recording sheet S.

[0069]Further, the second patch sensor 44 is disposed below the
intermediate transfer belt 8 in a manner opposed thereto. Further, on the
recording sheet conveying path, a sheet sensor 45 is disposed for
detecting the recording sheet S.

[0070]As the intermediate transfer belt 8 rotates and the recording sheet
S is conveyed, the image leading edge-detecting patch 42 is detected by
the second patch sensor 44, and based on the timing of detection of the
image leading edge by the second patch sensor 44 and the timing of
detection of the recording sheet S by the sheet sensor 45, the conveying
speed of the sheet S is varied to thereby cause the image leading edge
and the leading edge of the recording sheet to register with each other.

[0071]Before executing this control, first, control is performed to cause
the density of the image leading edge-detecting patch 42 (hereinafter
also simply referred to as "the patch") to have an appropriate value.
First, the second patch sensor 44 sequentially detects patches the
density of which is uniformly varied between 100% and 10%, e.g. in a
sequence of 100%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10%.

[0072]At this time, the output voltage of the second patch sensor 44
varies with the density of the patch as shown in FIG. 4. Through
comparison of the output voltage of the second patch sensor 44 with a
reference value, a patch exhibiting a density value which is second
highest to the density value which has been the first to fall outside the
appropriate range is used as the image leading edge-detecting patch 42.

[0073]In the sequence of adjusting the patch density is, as shown in FIG.
6, first, a patch having a density of 100% is formed. The second patch
sensor 44 detects the patch, and it is determined whether or not the
output voltage of the second patch sensor 44 at the time is within the
appropriate range.

[0074]If the output voltage indicative of detection of the patch having a
density of 100% is not within the appropriate range, it is determined
that a sensor error has occurred, whereas if the same is within the
appropriate range, a patch having a next lower density is formed. Then,
the second patch sensor 44 detects the patch again, and the output
voltage indicative of detection of the patch is compared with the
appropriate range, and if the output voltage is outside the appropriate
range, the patch density of the patch formed on the immediately preceding
loop is used for leading edge registration control.

[0075]If the output voltage is within the appropriate range, a patch which
is further lowered in toner density is formed. Thus, the patch density
associated with the output voltage which is highest within the
appropriate range is used for the leading edge registration control.

[0076]In the above-described sequence, the patch density is lowered
whenever the second patch sensor 44 detects a patch density once, and
hence it takes time to complete the patch density adjustment.

[0077]Therefore, as shown in FIG. 7, patches having densities of 100% to
10% may be sequentially formed with a certain amount of gap between each
adjacent ones of them to form an image leading edge-detecting patch 42,
and the image leading edge-detecting patch 42 may be detected (waveform
61), as shown in FIG. 4, for toner density adjustment.

[0078]More specifically, patches the density of which is uniformly varied
between 100% to 10%, e.g. in a sequence of 100%, 90%, 80%, 70%, 60%, 50%,
40%, 30%, 20%, and 10%, with a certain amount of gap between each
adjacent ones of them are formed sequentially at equal space intervals.
Then, the patches are detected sequentially by the second patch sensor
44.

[0079]Then, as described hereinbefore with reference to FIG. 7, the number
of times of reading each patch is recorded, and at the same time, it is
determined whether or not the associated output voltage of the second
patch sensor 44 is within an appropriate range. If the output voltage
indicative of the density of a patch which is detected first is outside
the appropriate range, it is determined that a sensor error has occurred.
If the output voltage is within the appropriate range, it is determined
whether or not the output voltage indicative of the density of a patch
which is next lower than that of the first detected one is within the
appropriate range. If the output voltage at this time is within the
appropriate range, the output voltage indicative of the density of a
patch which is further lower is checked. After all, a patch which is
lowest in toner density on condition that the output voltage
corresponding thereto is within the appropriate range is used as the
image leading edge-detecting patch 42.

[0080]Although in the above-described embodiment, the patches are
generated starting with a patch with a density of 100% (density: 1.6),
but patches may be generated starting with a patch with a density of 1.4.
In this case, it is possible to detect patch densities using an even
smaller number of generated patches. Further, although the patches are
sequentially generated in the order of decreasing densities, they may be
sequentially generated in the order of increasing densities, whereby not
only the amount of toners consumed for forming the image leading
edge-detecting patches, but also the amount of toners consumed at a stage
of determining a patch density can be reduced, and further the burden on
the cleaners can be lessened.

[0081]In this case as well, without generating patches starting with a
density of 0.1, but by starting with a density e.g. 1.4 close to a
threshold where the value changes from outside an appropriate range into
the appropriate range, to progressively increase the density, it is
possible to detect patch densities by minimizing the number of generates
patches. In this case, a value of the density which results in a shift
from outside the appropriate range into the appropriate range, i.e. 1.0
is caused to be set to the density of a patch.

[0082]FIG. 9 is a functional block diagram of the color image forming
apparatus according to the present embodiment.

[0083]As shown in FIG. 9, the color image forming apparatus is comprised
of a generation unit 901 that generates a position-detecting patch, and a
detection unit 902 that detects the position-detecting patch.

[0084]Further, the color image forming apparatus is also comprised of a
determination unit 903 that determines whether or not the density of the
position-detecting patch is within an appropriate range, an adjustment
unit 904 that adjusts the density of the position-detecting patch, and a
selection unit that selects the position-detecting patch. As for the
adjustment unit 904 and the selection unit 905, only one of them may be
provided.

[0085]As described heretofore, according to the present embodiment, by
holding the densities of patches to the minimum, it is possible to reduce
the toner consumption amount and lessen the burden on the cleaner 12 of
the intermediate transfer belt 8. Particularly, the image leading
edge-detecting patch 42 is formed on each recording sheet, and hence it
is very effective in reducing the toner consumption amount.

[0086]While the present invention has been described with reference to an
exemplary embodiment, it is to be understood that the invention is not
limited to the disclosed exemplary embodiment. The scope of the following
claims is to be accorded the broadest interpretation so as to encompass
all modifications, equivalent structures and functions